The present invention relates to apparatus for wireless signal propagation and, more particularly, apparatus for allowing wireless signal propagation through walls.
Wireless communication systems, based on radio frequency (RF) or microwave frequency technologies, are becoming increasingly popular in replacing conventional wired systems, for example in machine monitoring, control and automation systems. Wireless signals have the ability to penetrate various mediums depending on their dielectric properties.
However it is known that wireless technologies face difficulties communicating directly through metal barriers. Under conditions where a direct line-of-sight between a transmitter and a receiver is inhibited by a metal barrier, transmission of a wireless signal typically relies on indirect propagation paths or, so-called, ‘scatter’ wherein signals are reflected off the surfaces of other mediums to reach the desired receiver.
Communication is physically impossible where there is no direct or indirect propagation path available between transmitter and receiver, such as when the two entities are located in separate, sealed metal compartments. Conventional techniques for penetrating electronic signals through a metal barrier between two compartments typically involve the use of traditional electrical cabling inserted via a conduit. However conventional electrical wiring and cabling through metal requires a significantly large diameter hole (typically 10's of millimetres) to be made through the barrier. This is often not possible due to design restrictions, for example in enclosures that are required to contain fluid or to be sealed and/or pressurized in use. Holes of such magnitude can also compromise the structural integrity of the barrier/wall being penetrated.
Furthermore, if one attempts to provide a blank hole in the metal barrier to join the airspace between the two different compartments, and thereby bridge the metal barrier, the diameter needed to accommodate wireless signals using RF and microwave frequencies (i.e. for industrial, scientific and/or medical uses) is typically also too large to provide a viable solution for many applications. The minimum hole diameter for conventional free-air wireless propagation through that hole is, for example, approximately 73 mm and above for 2.4 GHz or 35 mm and above for 5 GHz. Having such ‘large’ holes will have negative implications in complex metal structures that may be sealed, watertight, pressurized and/or contain oil, etc. in use.
Furthermore, existing holes for electrical wires and cabling are usually filled with insulated wire cores (i.e. more metal) which effectively partially fills the hole with barrier material.
Other relevant technologies that exist in this area of communications based in the acoustic frequency range (e.g. ultrasonic communications) or eddy currents. However, such techniques are inherently high-powered and only allow very low data rates. They also suffer reliability issues due to susceptibility to noise and therefore remain substantially unusable for many applications, especially in and around metallic equipment and machinery that suffer from mechanical noises, vibration and other random noises.
It is an aim of the present invention to provide apparatus which can allow propagation of wireless signals through barriers for a wider variety of applications, or else for applications in which the above-described techniques are unsuitable. It may be considered an additional or alternative aim to provide apparatus which mitigates one or more of the above described problems.